The expression of recombinant Trichoderma atroviride endochitinase ech42 (rech42) in Pichia pastoris driven by the constitutive PGAP promoter was tested at low (3%) and high (40%) dissolved oxygen tension (DOT) conditions. Maximum volumetric enzyme activities of 18,600± 1053 and 12,330±1000 U l^-1 were attained at 3% and 40% DOT, respectively. Whereas maximum specific enzyme activities were 89.4±4.6 y 88.8±4.0 U mg_protein^-1. We found that conjunction of low DOT and the overexpression of rech42 affected the cell size of the P. pastoris but no the ech42 specific enzyme activity.